Top OOPs Interview Questions and Answers (2023)

Object-oriented programming (OOP) is a paradigm that uses objects containing data in programming. It has become a fundamental part of software development. Many companies hire professionals who have a good knowledge of object-oriented programming.

In this article, we have listed some of the frequently asked OOPs interview questions for freshers and experienced candidates. These interview questions will familiarize you with the most important object-oriented programming concepts and help you ace your job interview.

Object-Oriented Programming is a programming paradigm that is built on the concepts of Objects and classes. A class is a blueprint or template to create more specific objects. An object can be considered a real-world instance of a class.

Top OOPs Interview Questions and Answers for 2023

Q1. What is Object-Oriented Programming?

Ans. Object-Oriented Programming(OOPs) is a computer programming model based on objects instead of functions and procedures. The individual objects are grouped into classes. In OOP, the data and the functions are closely bound together so that the data cannot be accessed by any other part of the code except that function. Object-oriented programming breaks down complex things as simple structures. This paradigm offers many concepts, such as inheritance, abstraction, encapsulation, and polymorphism.

Check out this OOPs explanation with an Analogy.

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Q2. Name the main features of OOPs.

Ans. The main features of Object Oriented Programming Language are:

• Classes and Objects
• Data Abstraction
• Encapsulation
• Inheritance
• Polymorphism

Q3. What is a Class in OOP?

Ans. A class is like a template or blueprint of a house. Just like using blueprint you can build as many houses and modify each of them with different interiors, similarly using a class one can use it to describe one or multiple objects. A class is a collection of variables and methods which are common to all objects of a particular kind.

Q4. What is an Object in OOP?

Ans. Each house you build (or instantiate) is an object, also known as an instance of class. An object is the basic unit of Object-Oriented Programming. The term “object” in object-oriented programming refers to a specific instance of a class which can be a combination of variables, functions, and data structures within in a class.

Explore: Data Structure Interview Question

Q5. What is the difference between a Class and a Structure?

Ans. The differences between a class and a structure are:

Class	Structure
Class is a collection of objects with common properties	The structure is a group of different data types
It is a reference data type	It is a value data type
A Class uses the keyword “class”	A Structure uses the keyword “struct”
Supports Inheritance	Does not support Inheritance
An object is created in the heap memory	An object is created in the stack memory
Members are private by default	Members are public by default
An instance of a ‘class’ is ‘object’	An instance of ‘structure’ is ‘structure variable’
Declaration syntax: class class_name{ data member; member function; };	Declaration syntax: struct structure_name{ type struct_element 1; type struct_element 2; type struct_element 3; . };

Also read: Difference between Class and Structure

Q6. Which are the widely used OOP languages?

Ans. Some of the commonly asked OOP languages include:

• Java
• Python
• C++
• Go
• C#
• Ruby

Q7. What are the advantages of OOP?

Ans. The advantages of OOP are:

1. Reusability of code –

In Object-oriented programming, we can write a class once and use it many times by creating the object for the class.

2. Code Maintenance –

The existing code can be easily maintained and modified. New objects can be created by making small changes to existing codes.

3. Data Redundancy –

If we require a similar functionality in multiple classes, we can write common class definitions for similar functionalities and inherit them.

4. Security –

Using data hiding and abstraction, we can filter out limited data to view or exposure. This helps in maintaining security.

5. Design Benefits

Designers go through an extensive design phase for object-oriented programs. This results in providing better designs and fewer flaws.

All these features enable programmers to achieve higher productivity, reduce maintenance costs, and provide better quality software.

Q8. What are the limitations of OOP?

Ans. The limitations of OOP are:

1. Object-oriented programming has a steep learning curve and it can take some time to gain a good understanding of it. It may be a little hard to understand the OOP code initially without the corresponding class documentation.
2. The size of the OOP programs may be larger than the programs created using procedure-oriented programming.
3. Object-oriented programs are usually slower as they need more instructions to be executed.
4. OOP programs can use a large amount of memory.
5. Intensive testing processes are required.

Also Read: Top Python Interview Questions and Answers

Q9. What is a constructor? Name the different types of constructors.

Ans. A constructor is a special type of method that is called to create an object. The name of the constructors is the same name as the class.

Different types of constructors in object-oriented programming are:

• Default constructor
• Static constructor
• Copy constructor
• Private constructor
• Parameterized constructor

Q10. Name some operators that cannot be overloaded.

Ans. Some of the operators that cannot be overloaded are:

Dot or Member access operator – ( . )

Scope resolution operator – ( :: )

Ternary operator – ( ? : )

Pointer to member operator – ( .* )

Object size operator – (sizeof)

Object type operator – (typeid)

Must read: New and delete operators in C++

Q11. What are the differences between procedural programming and object-oriented programming?

Ans. This is one of the most important OOPs basic interview questions.

The differences between procedural programming and object-oriented programming are:

Procedural-Oriented Programming	Object-Oriented Programming
Procedural Oriented Programming follows a step-by-step approach for dividing a task into a set of variables and routines through instructions.	OOP uses classes and objects to create models instead of functions and logic.
A program is broken down into small parts called functions.	A program is broken down into small parts called objects.
It follows the top-down approach.	It follows a bottom-up approach.
No modifiers are introduced in Procedural Programming.	OOP has access modifiers such as private, public, and protected.
It is less secure as it does not have any proper way of hiding data.	OOP is more secure as it allows data hiding.
Overloading is not possible.	Overloading is possible.
Does not have the Inheritance feature.	It has the Inheritance feature.
Not suitable for complex problems.	Suitable for complex problems.
Examples: C, Pascal, FORTRAN.	Examples: Java, C++, Python, C#, Ruby.

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Q12. What is Inheritance?

Ans. Inheritance is a fundamental object-oriented programming concept in which a new class is created that inherits the properties of the already existing class. It supports “code reusability” and reduces the length of the code. The code written in one class can be extended into another class. Inheritance has a base class, which is inherited by the derived class.

Different types of Inheritance:

• Single inheritance
• Multi-level inheritance
• Hybrid inheritance
• Multiple inheritance
• Hierarchical inheritance

Q13. What is Polymorphism?

Ans. Polymorphism is a concept in the object-oriented programming language that means ‘one function, multiple forms’. It performs different things according to the object’s class, which calls it. Polymorphism is applied to the functions or methods. It enables the object to decide which form of the function to implement at compile-time and run-time.

Different types of Polymorphism:

• Compile-time polymorphism (Method Overloading)
• Run-time polymorphism (Method Overriding)

Check out the top frequently asked Java OOPs interview questions – read our blog Java Interview Questions.

Q14. What is Data Abstraction? What are the levels of data abstraction?

Ans. Data abstraction is an important concept of object-oriented programming that displays only the important information and unnecessary details or implementation from the users. This concept helps in reducing programming complexity and efforts.

Data Abstraction has three levels:

• Physical Level (Lowest level of data abstraction): It tells how the data is stored in memory.
• Logical Level: It has the information that is stored in the database in the form of tables. The relationship among the data entities in relatively simple structures is also stored.
• View Level (Highest level of data abstraction): Only a part of the actual database is visible to the user.

Must Check: Polymorphism in C++

Must check: Difference between encapsulation and abstraction

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Q15. What is encapsulation?

Ans. In object-oriented programming, Encapsulation refers to wrapping up the data and the code that operates on that data in a single unit. It is a protective shield that allows data-hiding as the data specified in one class is hidden from other classes. Example: Class. A class consists of data and methods that are bundled together into a single unit. Many programming languages use Encapsulation in the form of classes.

Q16. What is hybrid inheritance?

Ans. Hybrid inheritance combines different kinds of inheritance, such as multiple inheritance, simple inheritance, and hierarchical inheritance. Hybrid inheritance is the combination of one or more types of inheritance. For example, a combination of simple inheritance and hierarchical inheritance. Therefore, hybrid inheritance is also called multipath inheritance.

Q17. How can we achieve data abstraction?

Ans. Data abstraction is achieved with abstract methods or abstract classes. 

Q18. What is a copy constructor?

Ans. Copy Constructor is a type of constructor. It initializes an object using another object within the same class. There are two types of copy constructors:

• Default Copy Constructor
• User-Defined Copy Constructor

Q19. What is a destructor?

Ans. A destructor is a member function that is automatically called when an object goes out of scope or is being destroyed. A destructor has the same name as the class with a tilde ~ sign as a prefix to it. Destructors never have any arguments.

Destructor Example: the destructor for class String is declared as ~String().

Q20. What are the access modifiers? Name the different types of access modifiers.

Ans. Access modifiers in OOP are keywords that set the accessibility of classes, methods, and other members. They determine the scope of the method or variables that can be accessed from other objects or classes. The different types of access modifiers in OOPs are:

• Private
• Protected
• Public
• Friend

Q21. Is it possible to call the base class method without creating any instance?

Ans. Yes, it is possible to call the base class without instantiating it is a static method and if the base class is inherited by some subclass.

Q22. What is static polymorphism?

Ans. Static polymorphism is a type of polymorphism that occurs during compile time. Overloading is an example of static polymorphism.

Q23. Is it possible to run a Java application without the implementation of the OOPs concept?

Ans. No, it is not possible to run a Java application without the implementation of the OOPs concept since these applications are based on the OOPs model.

Q24. What is dynamic polymorphism?

Ans. Dynamic polymorphism implies runtime resolution of function calls. This is implied by overriding, followed by inheritance in c++. Dynamic polymorphism occurs at runtime, is based on object orientation, and allows separation of implementation of interfaces and class hierarchies. Late binding, dynamic dispatch, or getting dispatch at runtime requires two things: virtuality and indirection, such as pointers and references. Overriding is a type of dynamic polymorphism.

Q25. What is method overloading?

Ans. Method overloading is an OOPs feature of OOPs. Whenever the arguments that have been passed differ, it allows giving the same name to more than one method in a class.Method overloading is the process of overloading methods with the same name but different parameters. C++ provides a way to overload this function. Method overloading allows users to use the same name for different methods, but the parameters passed to the methods must be different. The return types of the methods can be the same or different.

Also check: Difference Between Overloading And Overriding

Q26. Is it possible to run a C++ application without the implementation of the OOPs concept?

Ans. Yes, it is possible to implement C++ applications without OOPs

Q27. Explain the diamond problem in multiple inheritance?

The diamond problem is a complication that arises in object-oriented programming languages that support multiple inheritance. Let’s see an example:

1. Setup:
   • Suppose you have a base class A.
   • Classes B and C inherit from A.
   • Class D inherits from both B and C.
2. Problem:
   • If there is a method m() in class A and none in B and C, then there won’t be any ambiguity if this method is called by an object of class D since it would be inherited from A.
   • However, if the method m() is overridden in both B and C, then the object of class D will have ambiguity. It will be confused about which method to inherit: either from B or from C.

This is called the diamond problem because the class structure in this scenario looks like a diamond:

    A
   / \
  B   C
   \ /
    D

Languages like Java and C# do not support multiple inheritance of classes primarily due to this ambiguity. However, C++ supports multiple inheritance and uses virtual inheritance to solve this problem.

Q28. What is a virtual function? Why is it used?

In C++, a function is declared as virtual in the base class with the intention of being overridden or redefined in a derived class. When a base class pointer points to a derived class object and a virtual function is called through that pointer, the derived class’s version of the function is invoked. Without the virtual keyword, the base class’s function would be called, even if it’s overridden in the derived class.

It is used because :

• Achieve Runtime Polymorphism
• Ensure Consistency
• Provide a Base Implementation
• Work with Abstract Classes

Let’s see an example :

In the example above, even though the pointer shape is of type Shape, it points to an object of type Circle. When the draw method is called on this pointer, the Circle class’s draw method is invoked, thanks to the virtual keyword in the base class.

Without the virtual keyword, the base class’s draw method would have been called, which would not have given the expected behavior in the context of polymorphism.

Conclusion

Many developers use the OOPs paradigm at some point in their careers. This is why many developers face OOPs concept interview questions during their job interviews.

We hope these OOPs concepts interview questions will help you gain an in-depth understanding of the concepts and help you crack your programming interview confidently.